The metabolic disposition of the anticonvulsant primidone (Mysoline, Ayerst) has not been well studied in man, even though it is extensively converted in the body to active metabolites that contribute to its therapeutic and adverse effects. We propose (1) to examine the metabolic disposition of primidone in epileptic patients, giving special attention to the formation of active metabolites and to the hepatic and renal processes involved in the disposition profile, and (2) to determine whether there is significant inter- and intra-patient variation in disposition that can explain disparate therapeutic responses. The study will be conducted with the cooperation of clinical neurologists involved in epilepsy therapy and research. Critical drug disposition measurements will be made from the blood and urine of epileptic patients undergoing regimented therapy at a hospital-associated neurology clinic. Patients will be classified according to age, sex, therapeutic regimen, duration of therapy, and clinical evaluation of effectiveness of therapy. These parameters will be statistically correlated with pharmacokinetic characteristics of the disposition profile. To contend with the complications imparted by chronic drug therapy, many of the drug disposition measurements will be made following administration of a single isotopically-labelled dose of primidone incorporated in the indicated therapeutic regimen. Thus, the studies are designed to obtain the required data without interrupting regimented therapy.